i965: Implement GenerateMipmap directly, rather than using Meta.

i965: Implement GenerateMipmap directly, rather than using Meta.

Meta is awful and we'd like to stop using it.  Implementing this using
BLORP allows us to stop trashing a bunch of GL state every time.

This follows the structure of st_generate_mipmap().
compute_num_levels is lifted directly from there.

Improves performance in Gl41HdrBloom by about 11.794% +/- 1.01919% (n=3)
on Kabylake GT2 at 1280x720 (the difference seems much smaller at higher
resolutions).

v2 (idr): Don't try depth or depth-stencil blorp blits on Gen4 or Gen5
because it's not implemented yet.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>